1. Field of the Invention
This invention relates to a process of producing synthetic threads from a polyester-based polymer mixture. The threads produced are suitable for use as continuous threads or can be processed to obtain staple fibers.
2. Summary of the Related Art
Spinning modified polymers is known and described, for example, in EP-A-0,860,524. A mixing means for mixing a polymer melt with a low-viscosity, liquid or gaseous additive is known from EP-A-0,766,996. Processing polymer mixtures to obtain filaments is described in WO 99/07927.
Normally, the design of mixing systems is not critical. However, polymer mixtures for the purpose of increasing elongation when spinning threads at high speed require certain measures with respect to the mixing efficiency in order to obtain a homogeneous and finely dispersed mixture and at the same time provide for thermally stable processing.
When producing polymer mixtures in large extruder spinning plants or direct spinning plants in which the matrix polymer melt is forced into at least one spinning line and exposed to high dwell times, one must substantially prevent mixing faults, thread faults and processing faults to ensure a high quality end product In particular, when certain additives are used, the high thermal load in such plants leads to a breakdown of the polymer, which results in a undesirably high level of low-molecular breakdown products as well as visible discolorations in the end product. WO 99/57348 discloses polymer mixtures whose additives lead to an unacceptable yellowing of the textile thread. Even when adding a phosphorus stabilizer, although perfect spools were obtained at first in a pilot plant, the yarn appeared yellowish when produced in a production plant with a distinctly higher melt dwell time.
The present invention comprises an improved method of producing polyester threads. The threads produced according to the invention can be processed to obtain staple fibers. Flat or bulky textile threads can be produced from flat threads. When textile threads are desired, unwinding speeds between 2500 and 3600 m/min can been employed, depending on the titer to be produced. Such POY threads have elongations at break of 85 to 180%, which is advantageous for further processing in a stretching process or stretch texturizing process. While it is known from the prior art that increased unwinding speeds result in reduced elongation at break of the thread and the minimum elongation at break necessary for processing cannot be guaranteed. Only by modifying the polymer and, in particular, using specific polymer mixtures according to the invention can a satisfactory elongation at break be achieved at high unwinding speeds. We discovered, however, that the amount of additive to be added to obtain low elongations at break must be increased considerably with increased unwinding speed or under spinning conditions.
The present invention comprises a method of increasing the elongation at break for polyester-based spun threads in high-capacity spinning plants in an economically optimized way. At the same time, the elongation at break values required for further processing in a stretching process or stretch texturizing process are adjustable at high spinning speeds. Furthermore, improved spooling behavior and reduction of faults in the spooled thread is obtained, so that, for example, high uniformity is ensured when dyeing the textile thread upon further processing.
The method comprises splitting a first polyester melt stream into a second and third stream, mixing an additive polymer into the second stream, adding the mixture to the third stream, and extruding, wherein the ratio of viscosity of the additive polymer and the second stream is judiciously selected. The aforementioned benefits are achieved as a result of the method under the conditions described below.